Summary of Quarterly Results - Summary of Unaudited Quarterly Financial Information(Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Results Of Operations [Line Items]
Impairment losses		$ 100	$ 1,800	$ 53,900	$ 600		$ 9,900	$ 14,100
Gain Loss On Sale Of Real Estate	$ 800			$ 20,900	$ 2,000	$ 38,000			$ 1,270	$ 380	$ 259